Apparatus and method for driving and controlling electric consumers, in particular heat plugs

ABSTRACT

An apparatus for driving and controlling electrical loads, in particular glow plugs, is proposed which includes semiconductor switches which are assigned to the glow plugs and can be driven by a microprocessor, and also includes at least one measuring resistor and is characterized in that the microprocessor (17) is so designed that the glow plugs (RK) are switched on and/or off sequentially with time displacement for such a short time that a virtually continuous current rise or decrease is produced and/or in that, in order to detect an open circuit or a short circuit in any of the glow plugs (RK), the glow plugs (RK) are driven sequentially at any desired time interval for a very short time, preferably for 1 ms and the current flowing through the glow plugs (RK) is measured with the aid of the measuring resistor (R) and/or in that one or more glow plugs (RK) are driven simultaneously if a high-energy overvoltage occurs in the voltage supply of this apparatus.

BACKGROUND OF THE INVENTION

The invention is based on an apparatus for driving and controllingelectrical loads, in particular glow plugs. In a known apparatus of thistype, glow plugs of an internal combustion engine of a motor vehicle aredriven sequentially with a phase displacement. However, this type ofdriving has the disadvantage that each time a glow plug is switched on,the current rise can substantially decay before the next plug isswitched on. With short pulse lengths, it is also possible that a plugis already switched off again before the next plug is switched on. Thisproduces high-frequency interference in the vehicle supply system.

SUMMARY OF THE INVENTION

The apparatus according to the invention for driving and controllingelectrical loads and the method for driving and monitoring electricalloads by means of the apparatus have, on the other hand, the advantagethat negative effects on the voltage supply, when the electrical loadsor glow plugs are driven, are avoided by sequentially switching theloads on and/or off at short time displacements so that a virtuallycontinuous current rise or fall is produced. A particular advantage isthat the electrical loads or glow plugs are tested for open circuit orshort circuit by driving them in sequence at any desired time intervalwith measurement pulses of preferably 1 ms duration and determining thecurrent flowing through the glow plugs with the aid of the measuringresistor. It is particularly advantageous that high-energy interferencevoltages of the voltage supply or of the vehicle supply system arereduced by driving one or more glow plugs simultaneously for a certaintime.

It is particularly advantageous that the power of the individual loadsor glow plugs can be controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

Two exemplary embodiments of the invention are shown in the drawing andexplained in more detail in the description. In the drawing:

FIG. 1 is a schematic circuit diagram of the apparatus which includes amicroprocessor having a sequential logic circuit configured as a shiftregister,

FIG. 1a is a schematic circuit diagram of the apparatus wherein theshift register is included within the microprocessor,

FIG. 2 is a schematic circuit diagram of the apparatus according to FIG.1 having only one measuring resistor,

FIG. 2a is a schematic circuit diagram of an embodiment corresponding tothe embodiment of FIG. 1a except that only one measuring resistor isprovided and,

FIG. 3a shows a graph of the course of current for a first one of theglow plugs;

FIG. 3b shows a graph of the course of current for a second one of theglow plugs;

FIG. 3c shows a graph of the course of current for a third one of theglow plugs;

FIG. 3d shows a graph of the course of current for a fourth one of theglow plugs; and,

FIG. 3e shows the course of the voltage U_(R) across the resistor R ofFIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In principle, the apparatus is suitable for driving and controlling anyelectrical loads. Particularly advantageous, however, is the use fordriving and controlling glow plugs in motor vehicles having anautomatically controlled internal combustion engine. An exemplaryembodiment with four glow plugs is explained below.

For simplicity, FIG. 1 only shows the internal resistances RK of thefour glow plugs whose first end is connected to a first conductor 1connected to ground. Their second end is connected to a semiconductorswitch 3 which is connected via a shunt or resistor R, which acts as ameasuring resistor, to a second conductor 5. The conductor 5 isconnected to the voltage supply or the vehicle supply system, forexample to terminal 15, to which a voltage of, for example,approximately 12 to 14 V is applied during operation.

In the present case n-channel enhancement MOSFETs have been selected assemiconductor switches. Other semiconductor power switches can also beused. Source S and substrate or bulk B of the FETs are connected to eachother and are connected to the second end of the internal resistance RKof the glow plug which is situated opposite the ground connection. Thedrain electrode D of the FETs is connected to the node 7 at which thesemiconductor switches are connected to the measuring resistor R. Thegate electrode G is connected to a multistage sequential logic circuitwhich is shown here as a shift register 9. The subdivision of the shiftregister 9 into four sections indicates that each stage, that is, eachflip-flop, of the shift register is assigned to an FET 3. A measuringline 11 is connected from the nodes 7 to a signal evaluation or anundercurrent/overcurrent detection circuit 13 which determines thepotential present at the node 7 and compares it with the potentialpresent on line 5 and/or on line 1 by means of undercurrent/overcurrentcomparators. A signal line 15 is connected from the detection circuit 13to a microprocessor 17. The microprocessor 17 is connected via a drivingline 19 to the shift register 9.

FIG. 2 shows a further exemplary embodiment of the apparatus. In FIGS. 1and 2, corresponding elements are provided with identical referencesymbols.

FIG. 2 shows series connections of glow plugs, of which only theinternal resistance RK is shown for simplicity, and semiconductorswitches which are configured as n-channel enhancement MOSFETs 3. Thedrain electrodes D of all the FETs 3 are connected to each other at thenode 7. Between this node and the second conductor 5 there is, in thisembodiment, only one shunt or resistor R serving as measuring resistor.Owing to the change in the circuit, only one connecting line 11 isconnected to the undercurrent/overcurrent detection circuit 13.

FIGS. 3a to 3d show in separate diagrams the time-dependent course ofthe currents I_(K1) to I_(K4) flowing through the four glow plugs. Inaddition, in FIG. 3e, the course of the voltage U_(R) across theresistor R shown in FIG. 2 is shown. Finally, it is also shown at whatpoint in time the voltage is measured across the shunt or resistor. Thevoltage measurement is not required while the glow plugs are beingswitched off. This is made clear by the dotted representation.

The operation of the apparatus is explained in more detail below withreference to the FIGS.

During preheating, all the glow plugs are brought to a temperature ofapproximately 800° to 1000° C. For this purpose, the voltage supply,that is, the vehicle supply system has to deliver a high voltage. Thiscauses the vehicle supply system voltage to drop considerably if all theglow plugs are driven at the same time. High-frequency interferencevoltages occur in the vehicle supply system during phase-displaceddriving as described above. In the embodiments shown, the glow plugs aretherefore driven by the microprocessor 17 with time displacement. Thiscan be done by a suitable program stored in the microprocessor or by themicroprocessor having a multistage sequential circuit which, in thepresent case, is configured as shift register 9 and shown in FIG. 1a .FIG. 2a corresponds to FIG. 1a but shows the apparatus thereof with onlyone measuring resistor.

Each stage of the shift register 9 is assigned to an FET 3 which servesas a semiconductor switch. That is to say, the gate G of the FETs 3 isdriven by signals from the shift register 9 in such a manner that theFETs go over to the conducting state and thereby connect the glow plugsRK with the voltage-carrying conductor 5. The FETs 3 are driven in sucha manner that the glow plugs are sequentially switched on so rapidlythat, during switch-on, the current rise in one glow plug is still notentirely completed when the next glow plug is switched on.

In this way, a quasi-steadystate current rise is produced.

The process of switching off the glow plugs is controlled in acorresponding manner, that is, before the current decrease of a glowplug has decayed, the next one is switched off so that a virtuallycontinuous current decrease is produced. This results in a "damped"switch-off operation.

The preheating operation is consequently initiated and terminated insuch a manner that no high-frequency interference signals can beproduced in the vehicle supply system.

Faults in the glow plugs, for example, a short circuit or open circuit,can be detected by measuring the plug currents. The four resistors Rconnected in series with the FETs 3 and the internal resistances RK ofthe plugs serve this purpose according to FIG. 1. The voltage dropacross the resistors R is measured via the measuring lines 11 by theundercurrent/overcurrent detection circuit 13. This circuit evaluatesthe measured values preferably with undercurrent or overcurrentcomparators designed as individual comparators and delivers acorresponding output signal via the signal line 15 to the microprocessor17. The measuring lines 11 may also be connected to an OR-circuit whoseoutput signal is conducted to the detection circuit 13. The OR-circuitmay also be incorporated in the detection circuit 13.

FIG. 2 shows a simplification of the apparatus in which only a shunt ormeasuring resistor R is provided which is assigned to the parallelcircuit of all the plugs with the FETs 3. This likewise reduces thenumber of measuring lines 11 to one. Correspondingly, only onecomparator is provided in the detection circuit 13.

To detect open circuits, the plugs are switched on during vehicleoperation in sequence without heating at any desired time interval for avery short time, preferably for 1 ms. The current flowing through theplugs is measured by measuring the voltage drop across the shunt orresistor R. At the same time, it is not necessary to sample the voltagedrops across the resistors R individually in the detection circuit 13and to feed them to the individual comparators configured asundercurrent comparators; an OR-logic operation of the signals issufficient to determine whether a particular current threshold has beenexceeded or not. Both the embodiments in FIGS. 1 and 2 are suitable forthe undercurrent detection.

Because the plugs are driven by means of the microprocessor 17 via thecontrol line 19, it is known which plug has just been driven. In thisway, an open circuit, that is, an excessively low voltage or currentvalue, can be assigned to a plug without an identification occurringfrom the OR-logic operation.

During vehicle operation without glowing it is also possible to detectthe short-circuiting of a plug by measuring the voltage drop across theresistor R by means of individual comparators in the detection circuit13 configured as overcurrent comparators. As in the case of undercurrentdetection, the plugs are switched on sequentially at any desired timeinterval for a very short time, preferably 1 ms. Because of the knownassignment of the driving with respect to time by the microprocessor 17,an OR-logic operation of the measuring signals is also sufficient inthis case so that both exemplary embodiments can be used for overcurrentdetection. However, a higher current threshold should be chosen in thiscase than for the undercurrent detection.

The short-circuiting of plugs can also be detected during preheatingwhile the plugs are being switched on sequentially with timedisplacement. Owing to the assignment of the switch-on process withrespect to time, the defective plug can be identified if an overcurrentoccurs.

If short-circuiting of a plug only occurs when all the plugs have beenswitched on, an overcurrent or a short circuit can only be assigned to aparticular plug if an individual shunt is assigned to all the plugsaccording to FIG. 1.

If the measuring lines 11 in FIG. 1 are interconnected by an OR-element,the detection circuit 13 cannot detect which of the plugs isshort-circuited. In this case, all the plugs are first switched off andin a time-displaced switch-on process, a determination is then made asto which of the plugs is defective.

In the circuit according to FIG. 2, it is at first not possible todetermine which of the plugs is defective if the fault occurs after allthe plugs have been switched on.

Here, too, all the plugs are first switched off if an overcurrent occursand then the plugs are driven at any desired time interval with pulsesof preferably 1 ms duration with only one FET 3 being brought to theconducting state in each case. Since it is known which branch has justbeen energized when an overcurrent occurs, the defective plug can beidentified.

In the embodiment of FIG. 1, instead of the resistor R which serves asmeasuring resistor, the bulk resistance of the semiconductor switch canalso be used to measure the current flowing through the glow plugs. Inthat case, the potential present at the source electrode S has to bemeasured. Any other desired current measuring method can, however, alsobe used, for example, also Hall sensors.

The fault detection and identification of a defective plug can becombined with a visual and/or acoustic fault indication.

Defective plugs can be switched off selectively if a freely settablesequential circuit is used. In this way, interference in the vehiclesupply system can be avoided without it being necessary to shut off theengine immediately.

The apparatus according to FIGS. 1 and 2 are also suitable for reducinginterference voltages. In motor vehicles high-energy interferencevoltages, for example, so-called load-dump pulses may occur which assumea voltage of up to 120 V over several hundred milliseconds for aninternal resistance of 0.5 to 4 Ω. To suppress such pulses, which mayresult in the destruction of electronic control equipment, protectiveZener diodes have been used up to now which convert the energy of theinterference signal source into heat. Large and expensive diodes arenecessary for this purpose.

The energy of these interference signals can also be reduced orconverted into heat with suitable driving via the glow plugs.

For this purpose, the microprocessor 17 determines in any desired waywhether a fairly high interference voltage of, for example, 50 V andover is present. If this is the case, one or more glow plugs areswitched on simultaneously by a control signal delivered via the driveline 19, for example after 1 ms, preferably for 200 to 300 ms, to ensurethe reduction of the dangerous energy. The parallel-connected glow plugshave a total resistance of approximately 100 mΩ, so that theinterference source is so heavily loaded that the interference voltagedrops to a value which is safe for electronic control equipment.

In this way, interference voltages can only occur for approximately 1 msbefore the microprocessor 17 responds. These voltages can be reducedwith substantially smaller and less expensive protective Zener diodes.

The driving apparatus explained in more detail with reference to thefigures can also be used, as is evident from FIG. 3, to control thepower delivered by the glow plugs. When the glow plugs are switched onsequentially, the voltage dropping across the shunt or resistor R(compare with FIG. 2) common to all the plugs is measured. Thesequential driving of the plugs can be seen in FIG. 3 from the variationwith time of the currents I_(K1) to I_(K4) assigned to the individualplugs. Since a common shunt is assigned to all the plugs, the voltageU_(R) dropping across this resistor R, whose variation with time is alsoshown in FIG. 3, is proportional to the total current. According to FIG.3, the measurement of the voltage is shown in a separate diagram.

The instantaneous electrical power associated with each individual plugis calculated with the aid of the microprocessor 17 from the voltagechanges corresponding to the particular plug current and from theinstantaneous operating voltage.

A predetermined mean power can be set on the basis of this calculationfor each individual plug. This takes place because the switch-on timecan be lengthened or shortened by Δt. In FIG. 3, the switch-on time ofI_(K2) is shortened and that of I_(K3) is lengthened. In this way,variations in the tolerances of the plugs, which may lead to the currentlevel varying by ΔI, can be compensated for, as can the variations inthe vehicle supply system voltage and different cylinder performance.

Finally, it should further be pointed out that the driving apparatusdescribed can also be used for controlling the temperature of the glowplugs. For this purpose, for example, temperature-dependent resistorswhose measurement signals are fed to the microprocessor 17 are assignedto the glow plugs. The microprocessor 17 then drives the glow plugs withshort switch-on pulses approximately 1 s long in order to maintain thedesired temperature.

We claim:
 1. A method of driving and testing at least two glow plugs ofa diesel engine which are each switchable by a semiconductor switchconnected in series with a measuring resistor and drivable by amicroprocessor, the method comprising the steps of:detecting respectivecurrents flowing through the glow plugs by measuring the voltage dropacross the measuring resistor; driving the semiconductor switches in atime displaced manner one after the other so that the sum of thecurrents flowing through all of said glow plugs provides a quasi-steadycurrent rise as the switches are driven into their conductive state;and, determining the presence of an open circuit or a short circuit fromthe detected currents.
 2. The method of claim 1, wherein aninstantaneous electrical energy of the individual glow plugs isdetermined and the switched-on duration of each of the glow plugs isindividually shortened or lengthened for adjusting a predetermined powerof the glow plugs.
 3. The method of claim 1, wherein a switching deviceis provided for driving respective ones of said semiconductor switchesand an electrical supply is provided for the glow plugs, themicroprocessor and the switching device and a plurality of thesemiconductor switches are driven simultaneously when an energy-richovervoltage is detected in one of the following: the electrical supplyof the glow plugs, the electrical supply of the microprocessor and/orthe electrical supply of the switching device.
 4. The method of claim 1,wherein if the presence of an open circuit or a short circuit wasdetected after all glow plugs have been driven, switching off all of theglow plugs; and thereafter, again switching on the glow plugs in timedisplacement one with respect to the other for determining the defectiveglow plug.
 5. The method of claim 1, wherein said step of driving thesemiconductor switches comprises driving the glow plugs on sequentiallyat any desired time displacement one from the other for a very shorttime duration whereafter said step of detecting respective currentsflowing through the glow plugs is performed by measuring the voltagedrop across the measuring resistor for the purpose of detecting an opencircuit and/or a short circuit in one of the glow plugs.
 6. The methodof claim 5, said short time being preferably one millisecond.
 7. Anapparatus for driving and testing at least two glow plugs of a dieselengine, the apparatus comprising:a voltage supply; a plurality ofsemiconductor switches for switching on and off corresponding ones ofthe glow plugs; measuring resistor means connected in series with saidvoltage supply and said glow plugs; a microprocessor for driving saidsemiconductor switches in a time displaced manner one after the other;detection means for detecting a voltage across said measuring resistormeans to detect currents flowing through said glow plugs wherein thecurrents are utilized for detecting the presence of an open circuit or ashort circuit; and, said microprocessor including means electricallyconnected to said switches for driving said switches in a time displacedmanner so as to cause the sum of the currents flowing through all ofsaid glow plugs to provide a quasi-steadystate current rise as saidswitches are driven into their conductive state.
 8. An apparatus fordriving and testing at least two glow plugs of a diesel engine, theapparatus comprising:a voltage supply; a plurality of semiconductorswitches for switching on and off corresponding ones of the glow plugs;measuring resistor means connected in series with said voltage supplyand said glow plugs; a microprocessor for driving said semiconductorswitches in a time displaced manner one after the other; detection meansfor detecting a voltage across said measuring resistor means to detectcurrents flowing through said glow plugs wherein the currents areutilized for detecting the presence of an open circuit or a shortcircuit; and, said microprocessor including means for determining aninstantaneous electrical energy of an individual glow plug from saiddetected currents and for individually shortening or lengthening theswitched-on duration of each of the glow plugs for adjusting apredetermined power of the glow plugs.
 9. An apparatus for driving andtesting at least two glow plugs of a diesel engine, the apparatuscomprising:a voltage supply; a plurality of semiconductor switches forswitching on and off corresponding ones of the glow plugs; measuringresistor means connected in series with said voltage supply and saidglow plugs; a microprocessor for driving said semiconductor switches ina time displaced manner one after the other; detection means fordetecting a voltage across said measuring resistor means to detectcurrents flowing through said glow plugs wherein the currents areutilized for detecting the presence of an open circuit or a shortcircuit; said microprocessor including an electronic, multistageswitching circuit for driving said semiconductor switches; and, saidmultistage switching circuit being a shift register.
 10. The apparatusof claim 9, said shift register including a plurality of flip-flopscorresponding to respective ones of said glow plugs and said glow plugsbeing connected to corresponding ones of said flip-flops via respectiveones of said semiconductor switches.
 11. The apparatus of claim 10, saidshift register being provided for driving said semiconductor switchesand being realized by a program stored in said microprocessor.